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616 2
617
618 Technical Background
619
620 This chapter briefly introduces the key concepts necessary to understand inertial
621 confinement fusion (ICF), inertial fusion energy (IFE), and target physics.
622
623 INERTIAL CONFINEMENT FUSION AND INERTIAL FUSION ENERGY
624
625 Nuclear fusion—the process by which the nuclei of atoms such as deuterium or tritium
626 combine to form a heavier nucleus, such as that of helium—can release a significant amount of
627 energy. Fusion is the process by which energy is produced in the sun and, on a more human
628 scale, is the one of the key processes involved in the detonation of a thermonuclear bomb.
629 If this process can be tamed to provide a controllable source of energy that can be
630 converted to electricity—as the nuclear fission process is used in nuclear reactors—it is possible
631 that nuclear fusion could be a new way to produce low-carbon electricity to meet the growing
632 energy needs of the United States and the world. However, this possibility is far from imminent,
633 and a great deal of scientific and engineering work remains to be done before a commercial
634 nuclear fusion plant can be demonstrated.
635 For inertial fusion to occur in a laboratory, heating of the fuel material (typically
636 deuterium and tritium) must be confined to a small enough hot spot to overcome the Coulomb
637 repulsion of the nuclei and allow fusion to initiate in a small region of the fuel (“ignition”). If
638 successful, this process will release sufficient energy to sustain the fusion “burn” that will
639 propagate through the fuel, generating a significant energy output. Two concepts are typically
640 discussed for accomplishing this confinement: (1) magnetic confinement fusion (MCF), in
641 which magnetic fields are used to confine the plasma, and (2) ICF, the topic of the current report,
642 in which a driver delivers energy to the surface of a pellet of fuel, heating and compressing it.
643 Potential drivers include lasers, particle beams, and X-rays, among other concepts.
644 In ICF, energy supplied by the driver is applied, either directly or indirectly, to the outer
645 layer of a fuel pellet that is typically made up of an ablator material (e.g., beryllium, doped
646 plastic, or high-density carbon) that explodes outward as it heats. This outward explosion of the
647 surface layer forces the remainder of the fuel (typically light elements such as deuterium and
648 tritium) to accelerate inward to conserve momentum. The timing of the inward fuel acceleration
649 is controlled carefully in order to compress the fuel using a minimum of energy. At the same
650 time, sudden increases in the driver power profile both accelerate the implosion and send shock
651 waves into the center of the fuel, heating it sufficiently that fusion reactions begin to occur.9
652 The goal of ICF is to initiate a self-sustaining process in which the energetic alpha
653 particles emitted by the ongoing fusion reactions heat the surrounding fuel to the point where it
654 also begins to undergo fusion reactions. The percentage of fuel that undergoes fusion is referred
655 to as the “burn-up fraction.” The fuel gain G (defined as the ratio of the total energy released by
656 the target to the driving beam energy impinging upon it) depends on the burn-up fraction, and
657 gains greater than about 10 will need to be demonstrated to validate the target physics of any
658 approach to a practical IFE power plant.
9
What is described here is known as hot-spot ignition; other potential concepts for ignition are being considered,
and are introduced briefly later in this chapter.
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659 Important target physics includes processes that deflect or absorb driver energy within the
660 target; the transport of energy within the target; capsule preheat; conversion of energy to the
661 inward-directed implosion by ablation; fuel compression and heating; thermonuclear reactions;
662 transport and deposition of neutron and alpha-particle energy resulting in bootstrapping
663 thermonuclear reactions; and hydrodynamic disassembly and output. Models exist for all of these
664 processes, but some are more predictive than others. Some processes are difficult to simulate,
665 such as laser-plasma interactions, the generation and transport of hot electrons in self-consistent
666 magnetic fields, nonlocal-thermal-equilibrium atomic physics, hydrodynamic instabilities, mix,
667 and debris generation. These models continue to evolve to keep pace with experiments. Other
668 processes, such as large-scale hydrodynamics, thermonuclear reactions, and X-ray-, neutron- and
669 alpha-particle transport appear to be simulated adequately using standard numerical models.
670 The Department of Energy (DOE) is funding multiple efforts to investigate the physics of
671 ICF; many of these efforts have the potential to inform current understanding of the prospects for
672 IFE. Over the next several years, experiments will be ongoing at the National Ignition Facility
673 (NIF) at Lawrence Livermore National Laboratory (LLNL) that are aimed at achieving ICF
674 ignition. At the same time, experiments such as those at the University of Rochester's Laboratory
675 for Laser Energetics, the Naval Research Laboratory, Lawrence Berkeley Laboratory, and Sandia
676 National Laboratory continue to advance our understanding and control of ICF using different
677 technology and physics approaches. However, it should be recognized that up to this point, the
678 majority of the funding and efforts related to ICF target physics are provided by—and related
679 to—the U.S. nuclear weapons program and its stockpile stewardship efforts and are not directly
680 aimed at energy applications.
681 The DOE’s Centurion-Halite program revolved around a series of underground
682 experiments conducted in the 1980s in which target capsules were driven by the energy from
683 nuclear explosions. Additional discussion of the program is provided in classified Appendix D.
684
685 BASICS OF ICF TARGET PHYSICS AND DESIGN
686
687 Target Design: Direct and Indirect Drive, Z-pinch
688
689 There are two major concepts for ICF target design: direct-drive targets, in which the
690 driver energy (e.g., in the form of laser beams, particle beams, or magnetic field pressure)
691 directly strikes the fuel capsule (see Figure 2-1); and indirect-drive targets, in which the driver
692 energy first strikes a hollow chamber (a “hohlraum”) surrounding the fuel capsule, producing
693 energetic X-rays that compress the fuel capsule (see Figure 2-2). Conventional direct and indirect
694 drive share many key physics issues, such as energy coupling, the need for driver uniformity, and
695 hydrodynamic instabilities; however, there are issues that are unique to each concept.
696 Generally, the elements of the fuel capsule are similar for direct drive and indirect drive,
697 at least with respect to laser drivers. Fuel capsules are typically spherical, with several layers: an
698 outer ablator layer; a layer of cryogenic frozen fuel; and a center of gaseous fuel, typically
699 deuterium-tritium (D-T). A sample fuel capsule is shown in Figure 2-3.
700
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701
702 FIGURE 2-1 In the case of dire drive, the fuel pellet i illuminate symmetric
E ect e is ed cally by the
703 driver en
nergy, resulti in implos
ing sion. SOURC R. Betti University of Rocheste presentat
CE: i, y er, tion
704 to the NR IFE comm
RC mittee titled “Tutorial on the Physic of Inertial Confinemen Fusion,” o
d n cs nt on
705 April 22, 2011.
,
706
707
708
709 FIGURE 2-2 In the case of indi
E irect drive, driver energy incident on a hohlraum is converte to
d y n m ed
710 X-rays, which then im
w mpinge sym
mmetrically on the fuel ca
o apsule, causi it to implode. This fi
ing igure
16
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711 shows the laser beam geometry used in the National Igni
m u N ition Campaign (NIC) at the Lawren
t nce
712 Livermor National Laboratory. LEH, laser entrance hol LPI, laser
re L e le; r-plasma inteeractions; HDDC,
713 high-dennsity carbon. SOURCE: J. Lindl, LLN presenta
J NL, ation to the p
panel titled “
“The Nationnal
714 Ignition Campaign on NIF and It Extension to Targets f IFE,” on February 16 2011.
C n ts for 6,
715
716
717 FIGURE 2-3 Sectio of a spher
E on rical fuel cap
psule design showing the ablator lay (in this ca
e yer ase
718 pure carb
bon), a layer of DT ice, and an inner core of DT gas. Source: J. Lindl, LL
a LNL,
719 presentat
tion to the pa titled “T National Ignition Ca
anel The l ampaign on N and Its Extension to
NIF o
720 Targets for IFE,” on February 16 2011.
f 6,
721
722 Several of the key differe
e ences betwee direct driv and indire drive for ICF are
en ve ect
723 discussed briefly in the sections that follow.
d t t
724
725 Direct Drive
D
726
727 Direct-drive concepts for ICF using la drivers are currently being researched at the
D c aser y e
728 Universit of Roches
ty ster’s Labora atory for Las Energetic (LLE) and the Naval Research
ser cs d
729 Laborato (NRL). Concepts usin heavy-ion beam drive are being studied at L
ory C ng n ers g Lawrence
730 Berkeley National La
y aboratory (LLBNL), and Sandia Natio
S onal Laborat tories (SNL) is developing
)
731 direct-dri concepts for pulsed-p
ive s power driver rs.
732 The major benefit of direct-drive targ design is the calculate potential for higher
T get ed
733 energy ga than to in
ain ndirect drive This relati
e. ively large g is in larg part due to avoiding th
gain ge o he
734 losses tha occur duri the conv
at ing version of las beams or particle bea to X-ray in the
ser r ams ys
735 hohlraum discussed in detail in the next sect
m, t tion. Avoidin these loss results in a higher
ng ses n
736 percentag of driver energy absorbed by the capsule in d
ge direct drive, t
thus increasi the effici
ing iency
737 and potenntially decre
easing the siz of the driv required.
ze ver
17
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738 Polar direct drive is a var
d riant of the sp
pherically sy
ymmetric, diirect-drive il
llumination
739 geometry shown in Figure 2-1. As shown in Figure 2-4, t driver be
y F A the eams are cluustered in one or
740 two rings at opposing poles. To increase the uniformity o the drive, polar drive beams strike the
s g i of e
741 capsule obliquely, an the driver energy is bi
o nd r iased in favo of the mor equatorial beams.
or re l
742 Although the polar il
h llumination geometry is consequentl less efficie than the spherically
g ly ent
743 symmetric geometry, it is more compatible with the curr
c w rent NIF con
nfiguration.
744
745
746
747 FIGURE 2-4 In the polar direct
E t-drive illum
mination geom river beams are incident from
metry, the dr
748 direction above and below the fuel capsule but not near the equator. SOURCE: R. L. McCrory,
ns fu b .
749 Universit of Roches
ty ster, presenta
ation to the panel titled “
p “Laser-Drive Inertial Fusion Energ
en gy:
750 Direct-Drive Targets Overview,” on Februar 16, 2011.
s ” ry
751
752 Since the 198 there has been an on
80s, ngoing effort in laser scie
t ence that has been focus sed
753 on impro oving the perrformance of direct-drive laser system for both solid-state a KrF lase
f e ms and ers.
754 For solidd-state lasers, these advannces include frequency t
e tripling (for improved en nergy coupliing
755 and lowe instability growth rates), smoothin by spectra dispersion (SSD), and polarization
er ng al n d n
756 smoothin to reduce imprinting of beam non
ng, e nuniformitie on the targ Recently LLE developed
es get. y
757 SSD with multiple ph
h hase-modula ation frequen
ncies (Multi-
-FM) and pr roposed usin this techni
ng ique
758 to modify NIF for po direct dr
y olar rive.
759 High-energy KrF lasers were develop to utilize the deep ul
H w ped e ltraviolet (24 nm)
48
760 waveleng of the system. Induce spatial inc
gth ed coherence (I
ISI) was dev veloped to sm mooth the be eams,
10
761 and recen focal zo
ntly ooming was demonstrat to impro the effici
s ted ove iency of cou upling the las
ser
762 with imp ploding targe Direct-d
ets. drive target experiments on the OME
e EGA laser ha shown st
ave teady
763 improvem toward theoretical yield limits by combini a large n
ment ds l s ing number (60) of laser beam ms,
764 better las beam smo
ser oothing tech
hniques, and improved b eam pointin and target placement a the
ng at
10
Zooming involves redu
g ucing the drive spot size to match the diam
er m meter of the imp
ploding capsule thereby increasing
e,
the efficien of energy coupling betwe driver and target.
ncy c een
18
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765 target chamber center. Although historically much of the discussion of direct-drive fusion has
766 involved laser drivers (e.g., LLE’s work at the OMEGA laser facility and the Nike KrF laser
767 experiments at NRL), direct-drive ICF has potential for use with other drivers. In particular, the
768 panel was briefed on direct-drive targets by members of the LBNL heavy-ion driver program.
769 However, there are difficulties involved in using direct-drive fusion. A direct-drive
770 capsule must tolerate four major sources of perturbations to ignite and burn: drive asymmetry,
771 inhomogeneous capsule surface finish, ice roughness in the layer between the cryogenic DT and
772 the DT gas; and driver imprint.11 The effects of the driver imprint and drive asymmetry are
773 reduced for indirect drive. In addition, without a hohlraum to protect the capsule from the high
774 temperatures in the chamber, and if there is no buffer gas to protect the chamber walls from
775 emitted alpha particles, alternative methods must be found to address these threats.
776
777 Indirect Drive
778
779 As shown in Figure 2-2, indirect drive (whether using laser drivers or an alternative
780 driver, such as heavy-ion beams) consists of driver beams entering a hohlraum, which is
781 essentially a hollow cylinder, typically made of gold, or oblong capsule with (in the case of laser
782 drivers) openings on either end. LLNL is currently leading research into indirect-drive concepts
783 for laser-driven ICF at the NIF. The driver beams are directed to enter the openings on either end
784 of the hohlraum, and strike the interior of the hohlraum in four circular arrays, two near the
785 center, and two nearer the ends (see Figure 2-2). The energy deposited by the laser beams on the
786 interior of the hohlraum produces a hot plasma that radiates primarily in X-rays at a temperature
787 of about 300 eV or 3.3 million K. These X-rays are then absorbed by the capsule, resulting in
788 implosion.
789 A virtue of the hohlraum in an actual IFE target is that it functions as a thermal shroud to
790 protect the integrity of the cryogenic fuel capsule inside the target. This allows the target
791 chamber to contain an inert gas (xenon) at low pressure to help protect the walls of the target
792 chamber from X-rays emitted by high-Z materials in the exploding target.
793
794 Benefits of Indirect Drive for Smoothing
795
796 Spatial nonuniformities at any scale can significantly increase the deviation of the actual
797 implosion of an inertial fusion capsule from the conditions it was designed to achieve, with the
798 result that the conditions inside the imploded capsule lie in a less favorable location in
799 thermodynamic phase space than intended. Indirect drive of laser targets was conceived and
800 developed to eliminate the effects of nonuniformities within each laser beam delivered to the
801 target chamber.
802 The smoothing obtained through the use of indirect drive is a consequence of
803 transforming the energy of each laser from a focused beam into thermal radiation. Any
804 nonuniformity in a laser beam entering an indirect-drive target chamber transfers to the wall of
805 the hohlraum enclosing the target, heating its material to a heterogeneous plasma. This
806 heterogeneity is somewhat smoothed by energy transport processes within the radiating plasma
807 itself, but a stronger smoothing effect occurs because the X-rays originating in each localized
11
For laser drivers, driver imprint occurs early in time when the target ablator is cold and dense. It is related to the
asymmetries from modulations in individual laser beams (short wavelength) and perturbations from overlapping
drive beams or by beams with slightly differing arrival times and angles of incidence (longer wavelength).
19
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808 mass of plasma affect the entire portion of the target capsule surface to which it has a direct line
809 of sight. The result is that localized variations in X-ray emission are averaged over the capsule
810 surface, and rapid changes of drive conditions over the surface of the capsule are eliminated.
811 The development and use of indirect drive was the primary focus of LLNL on the 10-
812 beam NOVA laser. This experience led to the development of the NIF indirect-drive
813 configuration, which is much more sophisticated, using 192 laser beams in inner and outer
814 clusters to control symmetry and pulse shape (see Figure 2-2).
815 Although the capsule absorption of X-rays is more efficient than the direct absorption of
816 laser light in direct-drive fusion, enough energy is lost in the heating of the hohlraum to
817 significantly reduce the efficiency of indirect-drive fusion relative to direct-drive fusion. This
818 results in lower calculated potential gains for indirect-drive fusion targets.
819 As with direct drive, although its primary development historically has been with laser
820 drivers, indirect drive has been used in IFE system designs with other drivers (e.g., heavy ions
821 and early Z-pinch schemes). The key is to deposit enough energy on the inner surface of the
822 hohlraum to produce a hot plasma that radiates thermal X-rays.
823 One of the key reasons that indirect-drive targets were developed is that ICF can model
824 on a laboratory scale some aspects of a thermonuclear explosion. This is highly useful for the
825 applications of ICF at the NIF at LLNL that are related to the long-term stewardship of the U.S.
826 nuclear stockpile. This motivation has been a key aspect in the development of the indirect-drive
827 approach for IFE, since one could leverage insights from better-funded weapons programs for
828 the less well funded energy programs. However, there remains debate about whether this
829 provides significant benefits for energy generation using ICF, and some argue that the indirect-
830 drive approach—if commercialized and distributed overseas—could increase the risk that
831 nuclear weapons knowledge and information will proliferate. This topic is analyzed in more
832 detail in the classified Appendix E and in Chapter 3.
833
834 Z-pinch Target
835
836 In recent ICF and IFE studies, Z-pinch targets are imploded by the pressure of ultrahigh
837 magnetic fields generated by high currents (e.g., 20-60 MA for ~100 ns) provided by pulsed-
838 power generators rather than by the ablation pressure generated by illuminating a capsule with a
839 high-power laser. While laser fusion capsules are typically spherical shells, Z-pinch targets are
840 typically conducting cylindrical shells containing DT fuel. Since magnetic field strength
841 increases inversely with the radius of the conductor in which the current flows (I/r), as long as
842 the driver has the appropriate electrical characteristics to deliver current to the increasingly high-
843 inductance target, the magnetic pressure (proportional to B2) continues to grow, accelerating the
844 cylindrical implosion and compressing the fuel. For appropriate design conditions, the DT fuel
845 can be heated to sufficient temperature to initiate fusion reactions and compressed to sufficient
846 areal density (bulk density ρ times fuel radius r) to trap emitted alpha particles and initiate
847 bootstrap heating.
848
849
850 Physics of Different Types of Ignition
851
852 Hot-Spot Ignition
853
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854 Hot-spot ignition, described briefly earlier in this chapter, is the most commonly
855 discussed and best understood method for achieving ignition. Hot-spot ignition refers to the
856 creation of a small central mass of fuel that is heated to temperatures sufficient to begin efficient
857 thermonuclear burn (~10 keV), surrounded by a larger mass of dense but colder fuel that has
858 sufficient areal density (>300 mg/cm2) to trap alpha particles and initiate bootstrap heating.12
859 The primary reason for utilizing hot-spot ignition is to minimize the driver energy
860 requirements. Heating fuel to 10 keV is energy-intensive, so the goal is to use the driver energy
861 to launch a series of shocks that simultaneously coalesce and heat only a small central mass to
862 fusion temperatures, while quasi-isentropically compressing the main fuel mass as close to the
863 Fermi-degenerate limit (the minimum energy state for high-density matter) as possible. The
864 energy deposited by fusion alpha particles rapidly heats the cold, dense main fuel, causing it to
865 reach thermonuclear burn conditions. The fusion burn terminates when the rapidly heated fuel
866 mass overcomes the inertia of implosion and explodes to lower densities and temperatures where
867 fusion reaction rates rapidly decrease (hence the term “inertial confinement”).
868 In order to use minimum driver energy, it is important to compress most of the fuel near
869 the Fermi-degenerate adiabat. At least four laser pulses are required to provide the compression
870 energy in a time-dependent fashion that is consistent with this goal. More, smaller pulses—or
871 even a continuous power profile—could also be used, but the four-pulse system is the easiest to
872 control and observe experimentally.
873
874
875 Fast Ignition
876
877 In FI, ignition is separated from the compression phase. The fuel is compressed (using
878 lasers or another driver) at a lower velocity than in hot-spot ignition. The goal is to create a fuel
879 mass that has at least the 300 mg/cm2 areal density required to capture alpha particles, but not the
880 DT temperature to initiate fusion burn. The energy to ignite a small portion of this compressed
881 fuel is provided by a high-intensity, ultrashort-pulse laser. For the correct conditions, the
882 thermonuclear burn propagates from this heated fuel volume into the rest of the cold, imploded
883 fuel.
884 The leading approach to fast ignition uses a hollow cone of high-density material inserted
885 into the fuel capsule so as to allow clean entry of this second laser beam to the compressed fuel
886 assembly (see Figure 2-5). The principle of fast ignition was first demonstrated at the Institute of
887 Laser Engineering in Osaka, Japan, in experiments performed on the Gekko-XII laser (Kodama
888 et al., 2002).
889
12
R.L. McCrory, University of Rochester, presentation to the panel titled “Laser-Driven Inertial Fusion Energy:
Direct-Drive Targets Overview,” on February 16, 2011.
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890
891
892 FIGURE 2-5 In this version of fast ignition, a short, hig
E s f gh-intensity laser pulse e
enters the cone of
893 a cone-an
nd-capsule assembly afte the fuel ca
a er apsule has been compres ssed by an earlier pulse,
894 producin a pulse of hot electron that initiat fusion. SO
ng ns te OURCE: Jua Fernandez LANL,
an z,
895 presentat
tion to the pa titled “I
anel Inertial Conffinement Fussion Targets at Los Alam Nationa
s mos al
896 Laborato
ory,” May, 20 011.
897
898
899 Shock Ig
gnition
900
901 Shock ignitio is yet anot
on ther variant on the theme of slowing the main fu implosion to
o e g uel n
902 minimize driver ener requirem
e rgy ments, adding one more d
g drive elemen to locally h a limite
nt heat ed
903 quantity of fuel to the ermonuclear burn condit
r tions, and th using alp
hen pha-particle d deposition too
904 propagate the burn wave into the assembled fuel mass. In shock ignit
w e f n tion, rather t
than using a
905 separate, high-intensity, ultrashor rt-pulse lase to heat the ignited volu
er e ume, a short high-intens
t, sity
906 “spike” is added to th end of the main drive pulse shape to launch a very strong shock into t
he e e g the
907 fuel. This inward-pro
s opagating sh hock collides with the ou
s utward-propa agating shoc constituted by
ck d
908 the growing region of high-densi fuel at the center, pro
o ity e oducing a sph herical shell of fuel at a
l
909 much hig gher tempera ature. The pr rinciple of sh
hock ignition has been d
n demonstrated in experim
d ments
910 on the OM MEGA laser at LLE (Be et al., 2007). Since th target has a smaller ra
r etti he s adius at the ttime
911 that the high-intensity spike is required to lau
h y unch the fina shock, it is energetical advantag
al lly geous
912 if the lase optics can accommod focal zoo
er n date oming or, allternatively, if the high-intensity spik ke
913 can come from a separate set of lasers with smaller intrin spot size An issue t arises w
e l nsic e. that with
914 shock ign nition is that the final, hi
t igh-intensity spike excee the thres
y eds shold for laseer-plasma
915 interactio which can interfere with the des
ons, sired effect (
(see further d
discussion in Chapter 4).
n
916
917 Z-Pinch Ignition
918
919 Z-pinch targe need to ac
Z ets chieve the sa overall fuel parameters—that is sufficient
ame s,
920 temperatu to initiat thermonuc
ure te clear burn an area mass density to i
nd s initiate alpha
a-particle
921 bootstrap heating of the remainin fuel mass. Since the ta
p t ng argets are ty
ypically cylinndrical, the
922 convergeence is only two-dimensi
t ional and it is more diffi
i icult to meet the ρr criter
t rion. Some ttarget
923 designs work on the hot-spot ign
w h nition princip in which a small cen
ple, h ntral mass is shock-heate to
ed
924 thermonuuclear tempeeratures.
925 Alternatively, in magnetiz
A zed-target fuusion (MTF) the fuel ma is prehea by an en
), ass ated nergy
926 source (e a laser be
e.g. eam) to plac it on a hig
ce gher adiabat. Field coils a placed ar
are round the tarrget
927 to provid a seed magnetic field throughout the fuel volu
de t t ume. The ma agnetized, pr reheated fuel is
l
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928 then imploded at a lower implosion velocity than is used in hot-spot ignition to minimize driver
929 energy requirements. The magnetic field is applied to inhibit fuel cooling during the slow
930 implosion process (i.e., inhibit cross-field transport). The higher initial adiabat allows the
931 magnetically insulated fuel to reach thermonuclear conditions at smaller convergence ratios. The
932 principle of MTF has not yet been successfully demonstrated. MTF is normally considered more
933 as an attempt to find an easier path to ignition rather than as a path to high yield and high gain,
934 but recent numerical simulations indicate that high-gain MTF is possible using cylindrical
935 implosions with a cryogenic DT layer (Slutz and Vesey, 2012).
936
937
938 What Determines the Degree of Fuel Burn and Gain
939
940 Fusion yield Y scales strongly with capsule absorbed energy (Y ~ E5/3), which implies
941 there is a strong premium on efficiently delivering energy from the driver to the capsule. Energy
942 must be absorbed symmetrically into the fuel to avoid instabilities. Each target design has
943 different transport and deposition issues:
944 • Indirect drive (e.g., in the NIC at the NIF) requires transport of lasers through a
945 background gas and delivery through laser entrance holes (LEH) in the hohlraum (see
946 Chapter 4). Most of the driver energy goes to heating the hohlraum wall and the dense
947 plasma blown off the wall, so the process is inherently inefficient.
948 • Direct drive simplifies transport and focusing issues, but it is critical to avoid the
949 generation of hot electrons (which cause fuel preheat) from laser-plasma interactions.
950 This method is more efficient because it is direct, but symmetry and deposition
951 physics are very important.
952 • Z-pinches require a direct electrical connection between driver and target through a
953 recyclable transmission line (RTL). As the target implodes and the Z-pinch
954 inductance increases, there may be potential loss regions. Because of the RTL, each
955 shot requires the replacement of substantial structure.
956 • Heavy ions are charged particles that are susceptible to plasma instabilities when they
957 are focused to the intensities required for ICF (>500 TW). Accelerators work best at
958 low currents, so achieving a high power requires high particle energies, which makes
959 their energy deposition range long. This complicates target design.
960
961 As noted above, fusion yield is calculated to scale as absorbed energy E5/3, so delivering
962 more energy to the target results in significantly higher yield. For the same driver energy, direct
963 drive delivers more energy to the fuel than does indirect drive. Implicit in this yield-scaling is the
964 fact that the increasing fusion energy output comes from burning more fuel. Burning more fuel
965 requires compressing more fuel to near Fermi-degenerate conditions, which requires more
966 energy to be absorbed by the target. Since most of the fuel mass is in DT at solid (ice) density,
967 more fuel mass means targets of larger radius. Larger target radius has the additional benefit that
968 it increases the inertial confinement time of the fuel mass (determined by the imploded fuel
969 radius divided by the sound speed) and increases the burn-up fraction of the DT fuel
970 disassembly. The burn-up fraction depends on the areal density of the fuel capsule:
971
972 fb = ρr/(ρr + β(T))
973
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974 where β(T) = 5.5-6.5 g/cm2 for optimal burn conditions. For a burn-up fraction greater than
975 about 1/3, ρr must be greater than about 3 g/cm2.
976 All designs try to use driver energy efficiently; thus, they implode a cold mass of fuel
977 isentropically and a small amount of fuel to high temperature—either by hot-spot ignition, fast
978 ignition, or shock ignition. Instabilities can limit the propagation of burn from the ignition region
979 to the remaining fuel. “Yield over clean” (YOC) is a measure of the deviation of experiments
980 from ideal simulations.
981
982
983 Spectrum Output
984
985 The fusion reaction determines the initial partitioning of energy into alpha particles, X-
986 rays, and neutrons. The spectrum of particles hitting the IFE target chamber wall is a function of
987 the intervening materials, whether from the hohlraum, support structures (e.g., RTLs), or
988 chamber fill gas.
989 Indirect-drive targets have high-Z materials in the hohlraum that emit copious X-ray
990 radiation. Xenon gas can be used to absorb these X-rays and mitigate chamber wall damage (see
991 Chapter 4). The xenon gas will get hot, but the hohlraum is believed capable of protecting the
992 cryogenic fuel as it transits the chamber.
993 Direct drive usually assumes a vacuum in the target chamber, because the fuel pellet
994 cannot be thermally insulated from a hot background gas. A shroud containing helium gas at low
995 pressure and temperature has been considered, although it presents many difficulties. Even
996 though the target is made of low-Z materials, there are still X-rays and ions that strike the wall
997 and deposit their energy very locally. Magnetic diversion of ions is being considered in some
998 designs to protect the chamber wall.
999 Z-pinch reactors would have yields above 1 GJ and RTL structures in the chamber.13 This
1000 can lead to debris and shrapnel. The RTLs also can contain substantial residual magnetic field
1001 energy, which needs to be accounted for in determining which particles hit the wall. Thick, Li-
1002 containing liquid walls can be used to protect the chamber surface from short-range ions,
1003 neutrons, and X-rays.
1004 Heavy-ion driver concepts are tending to use liquid walls and perhaps background gases.
1005 There do not appear to be any unique or particularly challenging aspects to the heavy-ion output
1006 spectrum as compared with laser direct-drive or indirect-drive systems.
1007
1008
1009 Target Injection and Fabrication
1010
1011 For energy to be produced in a fusion reactor, the target (which is the fuel source) will be
1012 obliterated. Thus, for IFE to produce a steady flow of energy, a steady supply of new targets
1013 must be introduced into the system. The more frequently the targets are introduced and converted
1014 into energy, the more power is produced; and similarly, the more energy that is available in each
1015 target, the more power is produced. It is the details of these targets, and how efficiently the
13
M. Cuneo et al., Sandia National Laboratories, presentation to the NRC IFE committee titled “Pulsed Power IFE:
Background, Phased R&D, and Roadmap, April 1, 2011.
24
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1016 energy is released, that distinguish the different concepts for IFE. These differences and
1017 technical challenges are discussed in detail in Chapter 4.
1018 How frequently targets can be introduced into the fusion reactor (the repetition rate) is
1019 determined by engineering practicalities of each fusion concept. The repetition rate for the
1020 concepts discussed here varies from 0.1 to 20 Hz. These values are calculated estimates; the
1021 technical challenges of delivering targets into the fusion chamber at these rates with the required
1022 precision, while preserving the integrity of the target, has been—in the absence of a
1023 comprehensive IFE program—only superficially addressed. Specific engineering concepts will
1024 require comprehensive testing to determine whether the proposed repetition rates, and
1025 subsequent power production, are feasible. Equally important is to understand whether any
1026 degradation to the configuration of the target during this injection process could reduce fusion
1027 performance below the calculated performance.
1028 Operating a fusion reactor at a repetition rate of 20 Hz will consume 1.728 million targets
1029 per day. No credible process for cost-effectively producing this number of targets has been
1030 developed. Current ICF experiments show that there is a technical path for manufacturing targets
1031 that meet critical specifications; whether this technical path is a viable method for mass-
1032 producing targets remains to be established. These considerations are discussed next.
1033
1034 Target Injection
1035
1036 For laser-driven IFE, the target injection process poses four challenges: accuracy and
1037 repeatability (both spatially and temporally) of target placement; ability to track the target, target
1038 survival, and clearing of the chamber. These challenges are discussed in the following
1039 paragraphs.
1040 A necessary condition for achieving the optimal energy output from each target is that the
1041 target be uniformly compressed by the laser beams. This requires the target to arrive at the same
1042 point in space and at the same instant as the multiple laser beams. For the direct-drive target, the
1043 target must be within 20 μm (rms between the centerline of laser beamlets to the centerline of the
1044 target). Concepts developed and tested as part of the High Average Power Laser (HAPL)
1045 program14 (see Box 4-2) showed that a surrogate target could be repeatedly placed within 10 mm
1046 of target chamber center, where a final engagement system does the final pointing. For the
1047 indirect-drive targets currently under development, the target is required to be within 100 μm of
1048 the focus of the laser beam,15 which appears to be within the capabilities of the system developed
1049 by the HAPL program; however, one difference between the direct- and indirect-drive
1050 approaches to fusion is that the indirect-drive approach has a higher gas pressure in the reactor
1051 chamber that may affect the repeatability of the injection process (Norimatsu et al., 2003). These
1052 are issues to be resolved in a technology development program.
1053 The second challenge is the ability to track the target to make real-time, minor
1054 corrections to the pointing of the laser beams at the target. Here technical progress was achieved
1055 during the HAPL program by demonstrating the ability to track a target moving at 5 m/s and to
1056 steer beams in real time so as to engage it with ± 28 μm accuracy (Carlson et al., 2007). The
1057 system has been designed assuming an injection velocity of 50 m/s.
14
J. Sethian, Naval Research Laboratory, presentation to the panel titled “The HAPL Program to Develop the
Science and Technologies for Direct-Drive Laser Fusion Energy,” September 20, 2011.
15
M. Dunne, LLNL, “LIFE Target System Performance,” presentation to the panel on July 7, 2011.
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1058 The third technical challenge is to preserve the target’s critical specifications until the
1059 moment of the implosion. The problems are significantly different in this case for direct- and
1060 indirect-drive targets. For indirect-drive targets, the surrounding hohlraum will provide thermal
1061 protection. However, laser access to the target is through thin membranes (<0.1 μm thick) at each
1062 end of the hohlraum, and these holes will allow a sizeable heat load (both radiative and
1063 conductive) to be delivered to the target. The radiation portion of this heat load is reduced by the
1064 presence of internal shields within the hohlraum, which will also disrupt convective cells, but the
1065 conductive heat load is unaffected and the target’s temperature is calculated to rise ~85 mK,
1066 which is less than the 100 mK ceiling specified in one system design.16 The benefit of these
1067 structures to the target’s preservation is appreciable; however, this benefit comes at the cost of a
1068 complex structure that needs to be built to high precision, and this precision must be maintained
1069 during the acceleration loads that the target experiences when it is injected into the reactor. These
1070 loads to the target assembly have been calculated and are stated to be acceptable.17
1071 For direct-drive targets, target survival is the major challenge. The exact heat load to the
1072 target is strongly dependent on engineering parameters such as the gas pressure in the reactor
1073 chamber, the time the target is inside and exposed to the environment, and the temperature of the
1074 reactor; heat fluxes in excess of 1 W/cm2 to the target will compromise the target’s performance
1075 (Tillack et al., 2010; Bobeica, Ph.D. thesis, Bobeica et al., 2005).
1076 Multiple strategies are envisioned for minimizing the heat load; two possibilities are to
1077 add protective layers to the outer surface of the target and to minimize the gas pressure in the
1078 reactor (Petzoldt et al., 2002). Testing such strategies is a critical step in determining the
1079 engineering feasibility of the laser direct-drive fusion energy option.
1080 Finally, it is necessary to clear the chamber of debris between shots. In the past, there has
1081 been a tendency to minimize this problem because the other issues appear so much more
1082 daunting. However, new concepts, higher repetition rates (with incrementally more mass injected
1083 into the chamber per unit time), and the possibility of increasing the gas pressure in the reactor to
1084 improve the durability of the reactor structure (high gas pressure will reduce the X-ray and ion-
1085 induced damage to the chamber wall) complicate the process of clearing the chamber.
1086 Concepts for injecting targets for pulsed-power fusion energy are radically different and
1087 less fully developed than their laser-driven fusion energy counterparts. The signature difference
1088 is that targets are consumed at a rate of 0.1 Hz and that the target is a more massive structure (up
1089 to 50 kg) that includes transmission lines that couple the power to the target.18 Removing spent
1090 targets and installing new targets will be done using automated machinery.19 While this process
1091 is conceptually feasible, there remain substantial engineering considerations that need to be
1092 resolved to determine whether this process can be completed within 10 seconds.
1093 The heavy-ion fusion energy concepts originated as a variation of laser-driven concepts
1094 in which the driver energy is supplied by heavy ions accelerated by a linear accelerator.
1095 Subsequently, a variety of target-design concepts have been proposed: an indirect-drive design
1096 (3-4 GeV Bi+1); polar direct-drive design (3 GeV Hg+1); and a single-sided direct-drive
16
Ibid.
17
Ibid.
18
M. Herrmann, Sandia National Laboratories, “Z-pinch Target Physics,” presentation to the panel on February 17,
2011.
19
M. Cuneo et al., Sandia National Laboratories, “The Potential for a Z-pinch Fusion System for IFE,” presentation
to the panel on May10, 2011.
26
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1097 configuration (90 GeV U+4).20 The target-design concepts use indirect-drive, direct-drive, and
1098 single-sided direct-drive configurations. The target injection challenges are similar for heavy-ion
1099 and laser-driven fusion: the indirect-drive target benefits from the thermal shielding provided by
1100 the hohlraum, while the direct-drive target remains vulnerable to the hostile environment of the
1101 reactor chamber. Beyond these commonalities with laser-driven fusion, no target injection
1102 concept specific to heavy-ion fusion has been proposed.
1103
1104 Target Fabrication
1105
1106 Before the targets can be injected into the reaction chamber they must be fabricated to
1107 tight tolerances, which requires a well understood and reliable process that is suitable for mass
1108 production. The mass fabrication challenges posed for the different types of targets vary
1109 significantly, although there are technologies common to many of the targets that will benefit all
1110 concepts for fusion energy. In this section, the key challenges are outlined for the production of
1111 these targets for laser drivers, pulsed power drivers, and heavy-ion drivers.
1112 Targets proposed for each of the fusion energy concepts have equal mixtures of
1113 deuterium and tritium as the fuel. This fuel is confined in a spherical capsule for the laser-driven
1114 concepts and most of the heavy-ion concepts or in a conical “X-target” (see Figure 2-6) or
1115 cylindrical structure (see Figure 2-7) for direct-drive heavy-ion fusion and pulsed-power fusion,
1116 respectively. Fabrication of the conical and cylindrical structures appears to be straightforward,
1117 though the exact specifications are not yet well defined or tested. Fabrication of the spherical
1118 capsules is complicated—partially owing to the design and partially owing to the tight tolerances
1119 and stringent specifications. Researchers making these targets for the ICF and the HAPL
1120 programs produced targets with specifications that are acceptable for the laser-driven fusion
1121 concepts; however, it remains to be demonstrated that the fabrication process can be scaled to
1122 satisfy the requirements of an IFE program.
20
B.G. Logan, Lawrence Berkeley National Laboratory, “Heavy-Ion Target Design” presentation to the panel on
July 7, 2011.
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1123
1124
1125 FIGURE 2-6 The heavy-ion-dri
E iven “X-targ concept. B, magneti field; CH, plastic.
get” . ic
1126 SOURCE B. Grant Logan, LBN “Heavy-I Target D
E: L NL, Ion Design,” pressentation to the panel on July
n
1127 7, 2011.
1128
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1129
1130
1131 FIGURE 2-7 The cy
E ylindrical maagnetized lin inertial fu
ner usion (MagL target co
LIF) oncept.
1132 SOURCE S.A. Slutz SNL, “Design and Sim
E: z, mulation of M
Magnetized Liner Inertia Fusion
al
1133 Targets,” presentation to the pane on May 10, 2011.
” el
1134
1135 Drive Targe
Indirect-D ets
1136
1137 The indirect-d
T drive targets proposed fo laser-driven IFE (e.g., in the LIFE point desig
s or E gn)
1138 are a mod dification of the target cu
f urrently used at the NIF. The fundam
d mental desig is the sam
gn me:
1139 DT fuel is contained inside a cap
i psule that is supported in
s nside a hohlrraum. Howev there are
ver, e
1140 differenc in both th capsule an the hohlra
ces he nd aum. The caapsule is a biilayered stru
ucture with an
n
1141 outer layer of high-de ensity carbon (diamond) and an inne layer of lo
) er ow-density hhydrocarbon
1142 foam. Th hohlraum is elliptical (rather than cylindrical a is the NIF target) and made from lead
he as F d
1143 rather tha gold. Add
an ditionally, in
nternal memb branes (“shin shields”) are introduc to preven the
ne ced nt
1144 capsule having a dire line of sig to the las entrance holes in the hohlraum. T capsule is
h ect ght ser e The
1145 postulate to be manufacturable using a com
ed u mbination of mmicrofluidic and vapor d
c deposition
1146 technique and the DT fuel is ad
es, D dded by drilli a hole 5 µ in diameter in the cap
ing psule and seaaling
1147 it once th fuel is inserted. Cooli the targe assembly l
he ing et liquifies the DT fuel, wh is wicke
hich ed
1148 into the foam layer to make a uni
f o iformly thick fuel layer. New technologies will be required to
k
1149 form the foam layer inside an exi
i isting capsul and those technologie need to be consistent with
le, e es e
1150 a credible mass-prod
e duction proce ess.
1151
1152 Drive Targets
Direct-D s
1153
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1154 The direct-drive target proposed for fusion energy bears a close resemblance to the
1155 direct-drive target that is proposed for experiments at the NIF.21 The fusion energy target is a
1156 spherical foam capsule that is slightly larger than the NIF direct-drive target. The outer surface
1157 of the foam capsule has a fully dense plastic overcoat (to retain the fuel) and a thin reflective
1158 metallic coating to reduce the radiative heat load to the ice. Additional outer layers may be
1159 needed to provide greater protection to the target when it is injected into the reactor chamber.
1160 The DT fuel is diffused into the plastic shell and the target assembly is cooled to form the
1161 uniformly thick ice layer.
1162 The manufacturing processes for both laser-driven target designs are scalable for mass
1163 production. However, it remains to be demonstrated that these processes can achieve the
1164 production yield required for a fusion plant given the specifications that are required. At this
1165 point, such processes are near,22 but have not yet been proven for mass production. Any changes
1166 in the target design to improve the implosion physics (resulting from experiments at the NIF) are
1167 likely to be dimensional changes that can be easily accommodated by the existing manufacturing
1168 process instead of changes in configuration that would require new technologies.
1169 Two of the targets designs that are proposed for the heavy-ion driven fusion concept use
1170 indirect- and direct-drive implosion symmetries, so the manufacturing challenges are the same as
1171 for laser-driven fusion targets. A third more recently proposed target design is a single-sided
1172 direct-drive concept where liquid DT fills an X-shaped volume (two cones joined at the apex, see
1173 Figure 2-6). No production method has been proposed, nor are any tolerances proposed for the
1174 design, although it appears this target will have similar constraints and technical challenges as
1175 the other targets.
1176 The pulsed-power fusion energy targets are distinctly different from the other fusion
1177 energy targets. There are multiple designs; one is a cylinder made from beryllium and filled with
1178 cryogenic D-T gas. This target will be straightforward to manufacture and is considerably less
1179 complex than the other target designs. However, the additional components that are needed to
1180 inject this target into a pulsed-power fusion reactor must be better defined to fully evaluate the
1181 technological challenges to making the entire target assembly.23
1182
1183
1184 Factors Most Likely to Determine the Cost of Targets
1185
1186 It is important to appreciate that the technologies for making most of the components of
1187 the targets exist already; targets are being successfully manufactured for the existing ICF
1188 program, and with a few exceptions, any changes to the target to adapt it for energy applications
1189 appear to be technically feasible.
1190 Much of the cost of the ICF target today is due to the quality assurance process, in which
1191 each target must be thoroughly evaluated because the yield of acceptable targets is so low. Any
1192 future IFE technology program will need to evaluate whether current technologies can (1)
1193 produce a more consistent product and (2) maintain the high production yield when scaled to
1194 mass production.
21
P.B. Radha, University of Rochester, “Polar-Drive Target Design,” presentation to the panel on July, 7, 2011.
22
J. Sethian, NRL,“The HAPL Program to Develop the Science and Technologies for Direct-Drive Laser Fusion
Energy,” presentation to the panel on September 20, 2011, and “M. Dunne, LLNL, “LIFE Target System
Performance,” presentation to the panel on July 7, 2011.
23
S.A. Slutz, SNL, “Design and Simulation of Magnetized Liner Inertial Fusion Targets,” presentation to the panel
May 10, 2011.
30
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1195 The material and production costs for manufacturing the targets appear to be acceptable
1196 and will benefit from the economies of large-scale production if a viable process is developed.
1197 The costs for developing the manufacturing process and constructing the manufacturing facilities
1198 are less predictable, with the latter depending strongly on the former. However, these are one-
1199 time costs that when amortized over the number of targets that are produced during the projected
1200 lifetime of the plant will likely be a small component in the cost of each target.
1201 A contributor to the cost of the target is the cost of the tritium fuel. Fusion energy has the
1202 appeal and requirement that tritium be bred in a reactor and be self-sustaining. Neutrons from the
1203 deuterium-tritium fusion process interact with a surrounding blanket of lithium/beryllium and
1204 produce proportional quantities of tritium. Once the plant is initially fueled with tritium, the cost
1205 of sustaining the fuel will be primarily the cost of extracting tritium from the by-products of the
1206 nuclear reaction and the cost of controlling the radiological hazards. (Deuterium, the other
1207 component of the fuel, is extracted from water.)
1208
1209
1210 Tritium Inventory Considerations
1211
1212 A consideration for selecting a target production concept, and possibly even a fusion
1213 energy concept, is the amount of tritium that is required to maintain the power plant in constant
1214 operation. While tritium-breeding will allow a facility to be self-sustaining, the complexity of
1215 recovering tritium from the breeder and reactor-chamber effluent, and then refueling the targets,
1216 will scale with the complexity of the operation and amount of tritium in the facility.
1217 Minimizing the amount of tritium in a power plant was an important consideration in
1218 designing the indirect- and drive-direct targets.24 More ambitious ideas were proposed for the
1219 indirect-drive concept that will require additional scientific and technical development to realize:
1220 drilling a hole in the target to add the fuel (and then resealing the hole) and achieving a
1221 uniformly thick fuel layer by suspending the fuel as a liquid within a foam layer. Combined,
1222 they would reduce the tritium inventory to less than 1 kg25 by recycling tritium through the
1223 facility in less than 8 hours. The first approach adds steps to the manufacturing process and
1224 should be technically feasible; the latter approach is also technically feasible, but it is unclear
1225 whether the liquid fuel can be cooled below its freezing point and still remain a liquid, which is
1226 what has to be done to achieve the gas density required in the capsule. If this is not possible, then
1227 an alternative and lengthier process is needed to form the ice layer, which would increase the
1228 tritium inventory.
1229 Minimizing the tritium inventory was a less important consideration for developing the
1230 direct-drive target. In any case, target tritium inventory for the direct-drive targets is much higher
1231 than for the current indirect-drive configuration. About 10 times more tritium is present in this
1232 target than in the indirect-drive target. Additionally, tritium is diffused into the capsule instead of
1233 flowing through a hole, which takes 2 to 4 days because of the fragility of the target and the
1234 quantity of fuel that has to be added.26 The process for forming the ice layer adds about 12 hours
1235 to the production cycle, which is the same process that the indirect-drive concept will use if it is
1236 not possible to subcool the liquid layer sufficiently to achieve the desired gas density.
24
M. Dunne, LLNL, “LIFE Target System Performance,” presentation to the panel on July 7, 2011.
25
M. Dunne et al., LLNL, "Overview of the LIFE Power Plant," presentation to the panel on April 6, 2011.
26
J. Sethian, Naval Research Laboratory, “The HAPL Program to Develop the Science and Technologies for Direct-
Drive Laser Fusion Energy,” presentation to the panel on September 20, 2011.
31
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1237 Two main contributors to the total tritium inventory of an IFE plant will be these:
1238 • The amount of tritium that is trapped inside the target during the target assembly
1239 phases and
1240 • The amount that is entrained in the tritium-breeding and recovery processes (from
1241 the gaseous effluent from the reaction chamber).
1242
1243 At this stage, there is insufficient information to know the optimum balance between
1244 these sources and whether the effort to minimize the amount of tritium in the target assembly
1245 process is worth the added manufacturing and technical complexities.
1246
1247
32
